Data communication receivers use digital demodulators to recover data from a received signal. A transmitter modulates a corresponding transmitted signal in a manner that conveys the data being communicated. A wide variety of modulation schemes are used in data communications, and many of these schemes, such as M-PSK and QAM, convey data through phase relationships between quadrature components of the transmitted signal. A digital demodulator in a data communication receiver operates in accordance with a selected modulation scheme to recover the data being conveyed.
Various non-coherent demodulation techniques are known. Non-coherent demodulation techniques do not rely on past data decisions in making current data decisions. These non-coherent techniques typically include squaring or higher order mathematical operations, such as raising signals to the fourth power for QPSK. Such operations are highly undesirable. They multiply the amount of Doppler which may be present in the received signal and correspondingly increase the noise level. Consequently, non-coherent demodulation techniques are often unacceptable for high Doppler or low signal-to-noise ratio (SNR) environments.
Coherent demodulation techniques provide desirable alternatives to non-coherent demodulation techniques for many low SNR environments. Coherent demodulation techniques use information derived from past data decisions in making current data decisions. Higher order mathematical operations may be avoided, and Doppler and noise influences in a received signal are not aggravated.
Unfortunately, prior art demodulators which use data decision feedback work satisfactorily only so long as either phase error between quadrature components experiences small change from symbol to symbol or a large amount of phase jitter may be tolerated. These are unreasonable restraints for many situations, such as when significant Doppler or other frequency offsets are present. Moreover, prior art data decision feedback demodulators tend to slowly acquire received signals. Inaccurate data decisions are likely during acquisition because little or no past decision data are available upon which to base current data decisions. Prior art data decision feedback demodulators feed these incorrect data decisions back so that the chances of making correct data decisions in the near future are low.